专业英语论文翻译
专业英语
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英文出处：https://www.sodocs.net/doc/665744346.html,/viewdoc/download?doi=10.1.1.132.1671&rep=rep1&type=pdf Daniel N. Liu and Michael P. Fitz
Department of Electrical Engineering
University of California Los Angeles, Los Angeles, CA, 90095
译文：
STBC-MTCM 译码算法复杂度在快速衰落信道的简化
摘要：STBC-MTCM 方案在多收发天线系统中是一个非常著名的传输分集方案，实现高速率，全分集和高的编码增益。在时间选择性快速衰落信道或频率选择性衰落信道中，现有的STBC-MTCM 译码器性能具有不能降低的误码底板。本文阐述两种计算有效的译码算法来抵偿快速衰落信道：归零（ZF ）检测器和线性最小均方误差检测器（LMMSE ）。所提出的译码算法在信道条件从准静态衰落到时间选择性快速衰落（TSFF ）或频率选择性衰落(FSF)变化时都有表现出了很好的特性。仿真结果表明我们提出的译码算法接近ML 算法的性能，同时具有较好的计算效率。 I.引言
在过去的十年里Telatar ，Foschini 和Gans 在无线通信系统多收发天线的理论容量方面进行了大量的研究。为了能够充分利用分集技术和接近多入多出系统的信道容量，多年来提出了各种空时码方案，如：分层空时编码结构（LST ），空时分组码（STBC ），空时网格码（STTC ），以及空时分组码与—多维网格编码调制级联的STBC-MTCM 。STBC 如文献[10]指出，空时分组码（STBC ）以非常低的编码复杂度就可以提供全分集，但是它不能获得编码增益；STTC 不仅提供了全分集而且还能获得编码增益，但它以较高的编码复杂度为代价。因此，用级联外码方法来折衷STBC 和STTC 的缺点是很自然的事，如在MTCM 的内部级联STBC ，这就是所谓的STBC —MTCM 的设计理念（这就产生了STBC-MTCM ）。在文[12]中，Siwamogsatham 和Fitz 通过扩展内码正交的STBC 码本，进一步加强了空时分组码原始速率损耗的STBC-MTCM 的设计。由于内部STBC 的正交性，在准静态（慢）衰落信道中可以采用复杂度低的ML 译码算法。
在所有已被提出的空时编码方案中，准静态分组衰落是广泛应用的前提假设。虽然这样的假设在很多的情况下均适用，但是时间选择性快衰落或频率选择性信道确实存在。例如，zhu 等人证明：在一个正常的高速公路上以h Km /110的速度行驶，可以导致归一化的多普勒效应扩展T f d 达到0.9%。在这种情况下，信道从一个符号到另一个符号将会有特别显著变化。因此，我们在STBC-MTCM 方案中取消了对准静态衰落信道假设的约束。
在快速衰落信道中，内部的STBC 的最佳译码复杂度激励着人们寻求低复杂度的译码算法。实际上,最佳译码器ML 内部的STBC 的复杂度是与吞吐量幂指数增长的。主要原因是由于主要原因是似然函数是联合函数，不能分解为单一的符
号函数之和的形式。为了解决复杂度的问题，我们提出了两种计算简单的次最优的检测：归零检测器ZF 和线性最小均方值检测器LMMSE 。由于STBC-MTCM 扩展码本的内部结构，一旦其他信号的子集属于另一信号的子集，滤波器的系数便可以通过酉变换来构造。本文所提出的译码算法在信道条件由准静态衰落到TSFF 或FSF 状态变化时都能很好的特性。而且，仿真结果表明，所提出的译码算法在快速衰落信道中接近ML 的特性，同时计算效率也更有效。
论文剩余部分的结构如下：第二部分概述了系统模型并引入相关符号，第三部分详细介绍了STBC-MTCM 低复杂度的译码算法，第四部分给出了STBC-MTCM 在快速衰落信道不同空间效率的几个计算例子。第五部分为论文的总结。 II.系统模型
A.外部编码器
我们考虑一个有t n 条传输天线和r n 条接收天线的空时多入多出（MIMO ）无线通信体系。这种传输方案在图1的上部给出了详细的介绍。值得注意的是：级联体系外码的级联可以是任意的（如BBC,LDPC 编码）。例如：Nm 的网格MTCM 编码的通常可作为一个外码，一个m t N N *的STBC 通常作为内码。假设长度为0Nk 的b 矢量作为源信息输入到码率为1/0k k Roc =的MTCM 编码器中。每k 段编码区间，编码一个0k 比特的数据字时会产生一个1k 比特的数据字，它被速率为Nm k Ric /1=的内部STBC 映射输出空时码字
C k ∈)(H 具有以下形
式： )]1()([)(-+???=Nm kNm x kNm x k X （1）
此时
????
????????=)(...)()()(21t x t x t x t nt x （2） }1.,1,{-+???+∈m m m m N kN kN kN t 。假设符号)(t x i 是与符号线性调制相对应的，并且是在时间t 第i 条天线传输的传输信号。此外，我们假设符号的平均能量|)(|2t x i Ex E ≡且信号在复杂的星座图中被选择的概率相同。总的谱效率是R 可以定义为m ic oc N k R R R /0==比特/信道利用位（BPCU ）。
图1 STBC-MTCM 体系
假设在信道中传输的相邻两个符号发生变化，且从一个天线到另一个天线的衰落是互相独立的。在接收端，假设完全同步，接收信号
Nm nr C k Y *)(∈可以
表示成 )]1()([)(-+???=Nm kNm y kNm y k Y （3）
特殊地，我们认为一个线性模型在符号瞬时时间t 接收的向量为1*1)](,),([)(nr T n C t y t y t y r ∈???=可以通过传输向量)(t x 表示为:
)()()()(t t t t y n x H += （4）
其中nt nr C t *)(∈H 是一个复杂的信道矩阵，在接收端是完全已知的，向量
1
*)(nr C t ∈n 是一个独立复杂的零均值高斯噪声，每个实分量的方差为
2/02N =δ。专门指出，)(k H 可以完全描述为:
)]()()([)(21t h t h t h t nt ???=H （5） 此时T nri i i i t h t h t h t h )]()(),([)(21???=，)(t h ji 则描述了在时间t 时从天线传输
端i 到天线接收端,...2,1,=j j 复杂的信道系数。我们定义信噪比0/SNR N E b =，b E 是每条接收天线上传输一位信息所需的能量。
B ．外部译码器
外部译码器的结构在图1下半部分给出了描述。由于我们采用了MTCM 的编码器，级联的STBC-MTCM 系统的译码过程可以分为两个阶段：并行转换过程和非并行转换过程（i.e. MTCM 译码）。一旦并行转换过程结束，接下来的任务就由Viterbi 算法直接给出。并行转换过程包括可能性计算和为每个非并行转换过程选择最可能的并行转换过程。由于信道的时间
选择性，文献[12]中简化的ML 检测方法对于并行转换过程不再是最理想的了。因为信号的正交性被毁坏以及似然函数在每个编码或译码区间内变成了联合的符号信息。
真正的ML 译码器为每个给定的网格分支寻找所有的并行转换。与文[12]中简化的ML 检测器不同，复杂的全开ML 译码器以吞吐量的指数倍增长和以||ρ为基数的并行转换。假
设在第k 段译码区间里，从输出状态u 到输入状态v 的分支度量uv T 计算为
21||);;()()(||)(min ∑-+=∈→-=Nm kNm kNm t p uv v u p t x t H t y k T ρ （6） 其中),;(v u p t x →定义为在瞬时信号时间t ，第p 个从状态u 到状态v 的并行转换向量。此外，最可能的并行转换),(k v u p ML
→∧从状态u 到状态v 在译码区间
k 里可以选为
),(k v u p ML
→∧=21||
);;()()(||arg m i n ∑-+=∈→-Nm kNm kNm t p v u p t x t H t y ρ (7)
为了简洁的讨论，我们定义),(k v u p ML →∧为ML p ∧。一旦计算出了所有的分支度量，Viterbi 的算法则可以用于寻找信道的最小累加度量。
英文：
Abstract —STBC-MTCM scheme which achieves high rate, full diversity and large coding gains is an outstanding example of transmit diversity scheme for multiple-antenna system. In the case of time selective fast fading (TSFF) or frequency selective fading (FSF) channels, the performance of current existing STBC-MTCM decoder suffers from an irreducible error poor. In this paper, we present two computational efficient decoding algorithms: zero-forcing (ZF) detector and linear minimum mean square error (LMMSE) detector to combat the fast fading channels. The proposed decoding algorithms provide a robust performance across range of channel conditions from quasi-static (slow) fading to TSFF or FSF. Simulation results suggest that our proposed decoding algorithms have near Maximum-Likelihood (ML) performance while being
computational more efficient.
I. INTRODUCTION
The seminal work by Telstar [1], Foschini and Gains [2] on theoretical capacity of multiple transmit-receive antennas in a wireless communication system has spawned a great deal
of research in the last decade. In order to fully capture the Benet of the available diversity and to approach the multiple input multiple-output (MIMO) channel capacity, many space-time coding schemes have been proposed over the years: layered space-time (LST) architecture [3]; space-time block codes (STBC) [4], [5]; space-time trellis codes (STTC) [6]–[8] and STBC-multiple trellis-coded modulation (STBC-MTCM) [9]. As pointed out in [10], STBC provides full diversity with very low encoder/decoder complexity; it does not provide any coding gain; while STTC provides full diversity as well as coding gain but at the cost of much higher decoder complexity. Thus, it seems natural to compromise the two extremes of STBC and STTC by concatenate an outer code such as MTCM [11] with an inner STBC. This resulted in the so-called STBC-MTCM designs. In [12], Siwamogsatham and Fitz further enhance the original rate-flossy STBC-MTCM designs by expanding the codebook of the inner orthogonal STBC. Due to orthogonality of inner STBC, a complexity advantage for ML decoding is available for quasi-static (slow)fading channels.
Among all these proposed space-time coding schemes [4]–[9], [12], quasi-static block fading has been a widely used assumption. While such an assumption can be met in fairly large number of scenarios, time selective (TS) fast fading [13], [14] or frequency selective (FS) [15] channels do exist. For example, Zhu et al. showed at a normal high way driving speed about 110 Km/h, which can induce normalized Doppler spread d up to 0.9% [13]. In such case, the channel may very signi?cantly from symbol to symbol. Thus, we relax the constraint of quasi-static fading channel assumption in decoding STBC-MTCM schemes. The complexity of optimal ML decoder on the inner STBC in fast fading channels motivates the search for low complexity decoding algorithms. In fact, the optimal ML decoder on the inner STBC has complexity that grows exponentially with throughput. The main reason is that the likelihood function becomes joint and no longer able to decompose into the sum of individual symbol functions. To address the complexity issue, we derive two computational simple suboptimal detectors: zero-forcing (ZF) detectors and linear minimum mean square (LMMSE) detector. Due to the inherent structure of the expanded cookbook in STBC-MTCM, later coefficients can be constructed via unitary transformation for every other signal subsets once it is formed within one subset. The proposed decoding algorithms provide a robust performance across a range of channel conditions from quasi-static (slow) fading to TSFF or FSF. Furthermore, simulation results suggest that the proposed decoding algorithms has a near ML performance in fast fading channels, while being computational more ef?cient.fast fading channels, while being computational more efficient.
The remainder of the paper is organized as follows: section II outlines the system model and introduces our notation. Section III details the reduced complexity decoding algorithms for STBC-MTCM. Section IV presents several numerical examples for different spatial efficiencies of STBC-MTCM in fast fading channels. Section V concludes the paper.
II. SYSTEM MODEL
A. Outer Encoder
We consider a space-time multiple-input multiple-output (MIMO) wireless communication system with t transmit and receive antennas. The transmission scheme is detailed in the upper half of
Fig. 1. Notice that the outer code for the concatenated system can be arbitrary (i.e. BCC, LDPC Code). For example: A MTCM encoder with trellis multiplicity of m is used as an outer code and a t × m STBC is used as an inner code. Let vector b with size 0 be source information bits entering the rate oc = 0 1 MTCM encoder. At the encoding interval, a 0-bit data word is encoded, yielding a 1-bit data word, which is then mapped by an inner STBC with rate ice = 1 m to output a space-time codewords.
for ∈{ m m+1 ··· m+ m?1}.We denote that symbol corresponds to the symbol linear modulated and transmitted on the th antenna at time . Moreover, we assume the average symbol
energy and symbols are equally likely chosen from a complex constellation. The overall spectral efficiency is then de?ned
bits per channel use (BPCU). The channel is assumed to be
varying from symbol to symbol of transmission and fades independently from antenna to antenna.
At the receiver side, assuming perfect synchronization, the received signal
Nm
nr
C
k
Y*
)
(
。
)]1()([)(-+???=Nm kNm y kNm y k Y （3）
In particular, we will consider a linear model at th symbol time instant in which the received vector
1*1)](,),([)(nr T n C t y t y t y r ∈???=T ∈ Cnr ×1 depends on transmitted vector
x( ) ∈ Cnt ×1 via )()()()(t t t t y n x H += （4）
where nt
nr C t *)(∈H is complex channel matrix, knownperfectly by receiver,
1
*)(nr C t ∈n is a vector of independent zero-mean complex Gaussian noise entries with
variance
2/02N =δ per each real component. More speci?rally,)(k H is fully described as
)]()()([)(21t h t h t h t nt ???=H （5） where T nri i i i t h t h t h t h )]()(),([)(21???=，)(t h ji sents the complex channel coef?cient from
transmit antenna to receive antenna ,
,...2,1,=j j ,at th symbol time instant. We also de?ne the signal-to-noise ratio (SNR) as 0/SNR N E b =，b E ,where b is the energy per transmitted information bit per receive antenna.
B. Outer Decoder
The structure of outer decoder is depicted in the lower half of Fig. 1. Since we are using MTCM encoder, the decoding process for the concatenated STBC-MTCM scheme can be divided into two stages: parallel transition processing and non-parallel transition processing (i.e. MTCM decoding). Once the parallel transition processing is ?nished, the later task is straightforward by Viterbi algorithm. The parallel transition processing includes likelihood computation and selection of the most likely parallel transition for each non-parallel transition (i.e. trellis branch). Owing to the channel’s t ime-selectiveness (i.e., H( t ) in (4)), the simpli?ed ML detection strategy in [12] for parallel transition processing is no longer optimal. This is due to the fact that signal orthogonality is destroyed and likelihood functions become joint over information symbols within each coding/decoding interval. The true ML detector inevitably restores to an exhaustive search over all parallel transitions for each given trellis branch. Unlike in the case of simpli?ed ML detector [12], the complexity of the Full-Blown ML detector grows exponentially with throughput and ||ρ which is the cardinality of parallel transitions. Assuming at the th decoding interval, the branch
metric uv T of outgoing state to incoming state is computed as:
21||);;()()(||)(min ∑-+=∈→-=Nm kNm kNm t p uv v u p t x t H t y k T ρ （6）
where ),;(v u p t x →denotes th parallel transition vector from state to state at th symbol time instant. Moreover, the most probable parallel transition
),(k v u p ML →∧ f
rom state to state at decoding interval is also selected: For brevity of discussion, from now on we will denote ),(k v u p ML
→∧ as ML
p ∧. Once
all branch metrics are computed, the Viterbi algorithm can be applied to search for the path with the smallest accumulated metric.
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外文翻译 英文原文 Belt Conveying Systems Development of driving system Among the methods of material conveying employed，belt conveyors play a very important part in the reliable carrying of material over long distances at competitive cost．Conveyor systems have become larger and more complex and drive systems have also been going through a process of evolution and will continue to do so．Nowadays，bigger belts require more power and have brought the need for larger individual drives as well as multiple drives such as 3 drives of 750 kW for one belt(this is the case for the conveyor drives in Chengzhuang Mine)．The ability to control drive acceleration torque is critical to belt conveyors’performance．An efficient drive system should be able to provide smooth，soft starts while maintaining belt tensions within the specified safe limits．For load sharing on multiple drives．torque and speed control are also important considerations in the drive system’s design. Due to the advances in conveyor drive control technology，at present many more reliable．Cost-effective and performance-driven conveyor drive systems covering a wide range of power are available for customers’ choices[1]. 1 Analysis on conveyor drive technologies 1．1 Direct drives Full-voltage starters．With a full-voltage starter design，the conveyor head shaft is direct-coupled to the motor through the gear drive．Direct full-voltage starters are adequate for relatively low-power, simple-profile conveyors．With direct fu11-voltage starters．no control is provided for various conveyor loads and．depending on the ratio between fu11-and no-1oad power requirements，empty starting times can be three or four times faster than full load．The maintenance-free starting system is simple，low-cost and very reliable．However, they cannot control starting torque and maximum stall torque；therefore．they are
《机械工程专业英语教程》课文翻译
Lesson 1 力学的基本概念 1、词汇： statics [st?tiks] 静力学；dynamics动力学；constraint约束；magnetic [m?ɡ＇netik]有磁性的；external [eks＇t?:nl] 外面的, 外部的；meshing啮合；follower从动件；magnitude [＇m?ɡnitju:d] 大小；intensity强度，应力；non-coincident [k?u＇insid?nt]不重合；parallel [＇p?r?lel]平行；intuitive 直观的；substance物质；proportional [pr?＇p?:??n?l]比例的；resist抵抗，对抗；celestial [si＇lestj?l]天空的；product乘积；particle质点；elastic [i＇l?stik]弹性；deformed变形的；strain拉力；uniform全都相同的；velocity[vi＇l?siti]速度；scalar[＇skeil?]标量；vector[＇vekt?]矢量；displacement代替；momentum [m?u＇ment?m]动量； 2、词组 make up of由……组成；if not要不，不然；even through即使，纵然； Lesson 2 力和力的作用效果 1、词汇： machine 机器；mechanism机构；movable活动的；given 规定的，给定的，已知的；perform执行；application 施用；produce引起，导致；stress压力；applied施加的；individual单独的；muscular [＇m?skjul?]]力臂；gravity[ɡr?vti]重力；stretch伸展，拉紧，延伸；tensile[tensail]拉力；tension张力，拉力；squeeze挤；compressive 有压力的，压缩的；torsional扭转的；torque转矩；twist扭，转动；molecule [m likju:l]分子的；slide滑动; 滑行；slip滑，溜；one another 互相；shear剪切；independently独立地，自立地；beam梁；compress压；revolve (使)旋转；exert [iɡ＇z?:t]用力，尽力，运用，发挥，施加；principle原则, 原理，准则，规范；spin使…旋转；screw螺丝钉；thread螺纹； 2、词组 a number of 许多；deal with 涉及，处理；result from由什么引起；prevent from阻止，防止；tends to 朝某个方向；in combination结合；fly apart飞散； 3、译文： 任何机器或机构的研究表明每一种机构都是由许多可动的零件组成。这些零件从规定的运动转变到期望的运动。另一方面，这些机器完成工作。当由施力引起的运动时，机器就开始工作了。所以，力和机器的研究涉及在一个物体上的力和力的作用效果。 力是推力或者拉力。力的作用效果要么是改变物体的形状或者运动，要么阻止其他的力发生改变。每一种
数学专业英语2-10翻译
Although dependence and independence are properties of sets of elements, we also apply these terms to the elements themselves. For example, the elements in an independent set are called independent elements. 虽然相关和无关是元素集的属性，我们也适用于这些元素本身。 例如，在一个独立设定的元素被称为独立元素。 If s is finite set, the foregoing definition agrees with that given in Chapter 8 for the space n V . However, the present definition is not restricted to finite sets. 如果S 是有限集，同意上述定义与第8章中给出的空间n V ，然而，目前的定义不局限于有限集。 If a subset T of a set S is dependent, then S itself is dependent. This is logically equivalent to the statement that every subset of an independent set is independent. 如果集合S 的子集T 是相关的，然后S 本身是相关的，这在逻辑上相当于每一个独立设置的子集是独立的语句。 If one element in S is a scalar multiple of another, then S is dependent. 如果S 中的一个元素是另一个集中的多个标量的，则S 是相关的。 If S ∈0,then S is dependent. 若S ∈0，则 S 是相关的。 The empty set is independent. 空集是无关的。 Many examples of dependent and independent sets of vectors in V were discussed in Chapter 8. The following examples illustrate these concepts in function spaces. In each case the underlying linear space V is the set of all real-valued function defined on the real line. V 中的向量的相关和无关设置的许多例子是在第8章讨论。下面的例子说明这些概念在函数空间。在每个 基本情况下，线性空间V 是实线定义的所有实值函数集。 Let 1)(),(sin )(,cos )(32221===t u t t u t t u for all real t. The Pythagorean identity show that 0321=-+u u u , so the three functions 321,,u u u are dependent. 321,,u u u 是相关的。 Let k k t t u =)(for k=0,1,2,…, and t real. The set ,...},,{210u u u S = is independent. To prove this, it suffices to show that for each n the n+1 polynomials n u u u ,...,,10 are independent. A relation of the form ∑=0k k u c means that
专业英语课文翻译
United 1 材料科学与工程 材料在我们的文化中比我们认识到的还要根深蒂固。如交通、房子、衣物，通讯、娱乐和食物的生产，实际上，我们日常生活中的每一部分都或多或少地受到材料的影响。历史上社会的发展、先进与那些能满足社会需要的材料的生产及操作能力密切相关。实际上，早期的文明就以材料的发展程度来命名，如石器时代，铜器时代。早期人们能得到的只有一些很有限的天然材料，如石头、木材、粘土等。渐渐地，他们通过技术来生产优于自然材料的新材料，这些新材料包括陶器和金属。进一步地，人们发现材料的性质可以通过加热或加入其他物质来改变。在这点上，材料的应用完全是一个选择的过程。也就是说，在一系列非常有限的材料中，根据材料的优点选择一种最适合某种应用的材料。直到最近，科学家才终于了解材料的结构要素与其特性之间的关系。这个大约是过去的 60 年中获得的认识使得材料的性质研究成为时髦。因此，成千上万的材料通过其特殊的性质得以发展来满足我们现代及复杂的社会需要。很多使我们生活舒适的技术的发展与适宜材料的获得密切相关。一种材料的先进程度通常是一种技术进步的先兆。比如，没有便宜的钢制品或其他替代品就没有汽车。在现代，复杂的电子器件取决于所谓的半导体零件. 材料科学与工程有时把材料科学与工程细分成材料科学和材料工程学科是有用的。严格地说，材料科学涉及材料到研究材料的结构和性质的关系。相反，材料工程是根据材料的结构和性质的关系来设计或操纵材料的结构以求制造出一系列可预定的性质。从功能方面来说，材料科学家的作用是发展或合成新的材料，而材料工程师是利用已有的材料创造新的产品或体系，和/或发展材料加工新技术。多数材料专业的本科毕业生被同时训练成材料科学家和材料工程师。“structure”一词是个模糊的术语值得解释。简单地说，材料的结构通常与其内在成分的排列有关。原子内的结构包括介于单个原子间的电子和原子核的相互作用。在原子水平上，结构包括原子或分子与其他相关的原子或分子的组织。在更大的结构领域上，其包括大的原子团，这些原子团通常聚集在一起，称为“微观”结构，意思是可以使用某种显微镜直接观察得到的结构。最后，结构单元可以通过肉眼看到的称为宏观结构。 “Property”一词的概念值得详细阐述。在使用中，所有材料对外部的刺激都表现出某种反应。比如，材料受到力作用会引起形变，或者抛光金属表面会反射光。材料的特征取决于其对外部刺激的反应程度。通常，材料的性质与其形状及大小无关。实际上，所有固体材料的重要性质可以概括分为六类：机械、电学、热学、磁学、光学和腐蚀性。对于每一种性质，其都有一种对特定刺激引起反应的能 力。如机械性能与施加压力引起的形变有关，包括弹性和强度。对于电性能，如电导性和介电系数，特定的刺激物是电场。固体的热学行为则可用热容和热导率来表示。磁学性质
机械类数控车床外文翻译外文文献英文文献车床.doc
Lathes Lathes are machine tools designed primarily to do turning, facing and boring, Very little turning is done on other types of machine tools, and none can do it with equal facility. Because lathes also can do drilling and reaming, their versatility permits several operations to be done with a single setup of the work piece. Consequently, more lathes of various types are used in manufacturing than any other machine tool. The essential components of a lathe are the bed, headstock assembly, tailstock assembly, and the leads crew and feed rod. The bed is the backbone of a lathe. It usually is made of well normalized or aged gray or nodular cast iron and provides s heavy, rigid frame on which all the other basic components are mounted. Two sets of parallel, longitudinal ways, inner and outer, are contained on the bed, usually on the upper side. Some makers use an inverted V-shape for all four ways, whereas others utilize one inverted V and one flat way in one or both sets, They are precision-machined to assure accuracy of alignment. On most modern lathes the way are surface-hardened to resist wear and abrasion, but precaution should be taken in operating a lathe to assure that the ways are not damaged. Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed. The headstock is mounted in a foxed position on the inner ways, usually at the left end of the bed. It provides a powered means of rotating the word at various speeds . Essentially, it consists of a hollow spindle, mounted in accurate bearings, and a set of transmission gears-similar to a truck transmission—through which the spindle can be rotated at a number of speeds. Most lathes provide from 8 to 18 speeds, usually in a geometric ratio, and on modern lathes all the speeds can be obtained merely by moving from two to four levers. An increasing trend is to provide a continuously variable speed range through electrical or mechanical drives. Because the accuracy of a lathe is greatly dependent on the spindle, it is of heavy construction and mounted in heavy bearings, usually preloaded tapered roller or ball types. The spindle has a hole extending through its length, through which long bar stock can be fed. The size of maximum size of bar stock that can be machined when the material must be fed through spindle. The tailsticd assembly consists, essentially, of three parts. A lower casting fits on the inner ways of the bed and can slide longitudinally thereon, with a means for clamping the entire assembly in any desired location, An upper casting fits on the lower one and can be moved transversely upon it, on some type of keyed ways, to permit aligning the assembly is the tailstock quill. This is a hollow steel cylinder, usually about 51 to 76mm(2to 3 inches) in diameter, that can be moved several inches longitudinally in and out of the upper casting by means of a hand wheel and screw. The size of a lathe is designated by two dimensions. The first is known as the swing. This is the maximum diameter of work that can be rotated on a lathe. It is approximately twice the distance between the line connecting the lathe centers and the nearest point on the ways, The second size dimension is the maximum distance between centers. The swing thus indicates the maximum work piece diameter that can be turned in the lathe, while the distance between centers indicates the maximum length of work piece that can be mounted between centers. Engine lathes are the type most frequently used in manufacturing. They are heavy-duty machine tools with all the components described previously and have power drive for all tool movements except on the compound rest. They commonly range in size from 305 to 610 mm(12 to 24 inches)swing and from 610 to 1219 mm(24 to 48 inches) center distances, but swings up to 1270 mm(50 inches) and center distances up
数学专业英语第二版-课文翻译-converted
2.4 整数、有理数与实数 4－A Integers and rational numbers There exist certain subsets of R which are distinguished because they have special properties not shared by all real numbers. In this section we shall discuss such subsets, the integers and the rational numbers. 有一些R 的子集很著名，因为他们具有实数所不具备的特殊性质。在本节我们将讨论这样的子集，整数集和有理数集。 To introduce the positive integers we begin with the number 1, whose existence is guaranteed by Axiom 4. The number 1+1 is denoted by 2, the number 2+1 by 3, and so on. The numbers 1,2,3,…, obtained in this way by repeated addition of 1 are all positive, and they are called the positive integers. 我们从数字 1 开始介绍正整数，公理 4 保证了 1 的存在性。1+1 用2 表示，2+1 用3 表示，以此类推，由 1 重复累加的方式得到的数字 1,2,3，…都是正的，它们被叫做正整数。 Strictly speaking, this description of the positive integers is not entirely complete because we have not explained in detail what we mean by the expressions “and so on”, or “repeated addition of 1”. 严格地说，这种关于正整数的描述是不完整的，因为我们没有详细解释“等等”或者“1的重复累加”的含义。 Although the intuitive meaning of expressions may seem clear, in careful treatment of the real-number system it is necessary to give a more precise definition of the positive integers. There are many ways to do this. One convenient method is to introduce first the notion of an inductive set. 虽然这些说法的直观意思似乎是清楚的，但是在认真处理实数系统时必须给出一个更准确的关于正整数的定义。有很多种方式来给出这个定义，一个简便的方法是先引进归纳集的概念。 DEFINITION OF AN INDUCTIVE SET. A set of real number s is cal led an i n ductiv e set if it has the following two properties: (a) The number 1 is in the set. (b) For every x in the set, the number x+1 is also in the set. For example, R is an inductive set. So is the set . Now we shall define the positive integers to be those real numbers which belong to every inductive set. 现在我们来定义正整数，就是属于每一个归纳集的实数。 Let P d enote t he s et o f a ll p ositive i ntegers. T hen P i s i tself a n i nductive set b ecause (a) i t contains 1, a nd (b) i t c ontains x+1 w henever i t c ontains x. Since the m embers o f P b elong t o e very inductive s et, w e r efer t o P a s t he s mallest i nductive set. 用 P 表示所有正整数的集合。那么 P 本身是一个归纳集，因为其中含 1，满足(a)；只要包含x 就包含x+1, 满足(b)。由于 P 中的元素属于每一个归纳集，因此 P 是最小的归纳集。 This property of P forms the logical basis for a type of reasoning that mathematicians call proof by induction, a detailed discussion of which is given in Part 4 of this introduction.
计算机专业英语课文翻译部分(第四版)
1.2 总线互连 总线是连接两个或多个设备的通信通路。总线的关键特征是，它是一条共享传输介质。多个设备连接到总线上，任一个设备发出的信号可以为其他所有连接到总线上的设备所接收。如果两个设备同时传送，它们的信号将会重叠，引起混淆。因此，一次只能有一个设备成功地（利用总线）发送数据。 典型的情况是，总线由多条通信通路或线路组成，每条线(路)能够传送代表二进制1和0的信号。一段时间里，一条线能传送一串二进制数字。总线的几条线放在一起能同时并行传送二进制数字。例如, 一个8位的数据能在8条总线线上传送。 计算机系统包含有多种不同的总线，它们在计算机系统层次结构的各个层次提供部件之间的通路。连接主要计算机部件(处理机, 存储器, I/O)的总线称为系统总线。系统总线通常由50~100条分立的(导)线组成。每条线被赋予一个特定的含义或功能。虽然有许多不同的总线设计，但任何总线上的线都可以分成三个功能组：数据线、地址线和控制线。此外可能还有为连接的模块提供电源的电源线。 数据线提供系统模块间传送数据的路径，这些线组合在一起称为数据总线。典型的数据总线包含8、16或32根线，线的数量称为数据总线的宽度。因为每条线每次传送1位，所以线的数目决定了每次能同时传送多少位。数据总线的宽度是决定系统总体性能的关键因素。 地址线用于指定数据总线上数据的来源和去向。例如，如果处理机希望从存储器中读一个字的数据，它将所需要字的地址放在地址线上。显然，地址总线的宽度决定了系统最大可能的存储器容量。 控制线用来控制对数据线和地址线的访问和使用。由于数据线和地址线被所有部件共享，因此必须用一种方法来控制它们的使用。控制信号在系统模块之间传送命令和定时信息。定时信息指定了数据和地址信息的有效性，命令信号指定了要执行的操作。 大多数计算机系统使用多总线，这些总线通常设计成层次结构。图1.3显示了一个典型的高性能体系结构。一条局部总线把处理机连接到高速缓存控制器，而高速缓存控制器又连接到支持主存储器的系统总线上。高速缓存控制器集成到连接高速总线的桥中。这一总线支持连接到：高速LAN、视频和图形工作站控制器，以及包括SCSI 和FireWire的局部外设总线的接口控制器。低速设备仍然由分开的扩充总线支持，用一个接口来缓冲该扩充总线和高速总线之间的通信流量。 PCI 外部设备互连是流行的高带宽的、独立于处理机的总线，它能够作为中间层或外围设备总线。当前的标准允许在66MHz频率下使用多达64根数据线，其原始传输速率为528MB/s, 或4.224Gbps。PCI被设计成支持各种各样基于微处理机的配置，包括单处理机和多处理机的系统。因此，它提供了一组通用的功能。PCI使用同步时序以及集中式仲裁方案。 在多处理机系统中，一个或多个PCI配置可通过桥接器连接到处理机的系统总线上。系统总线只支持处理机/高速缓存单元、主存储器以及PCI桥接器。使用桥接器使得PCI独立于处理机速度，又提供快速接收和传送数据的能力。 2.1 光存储介质：高密度存储器 2.1.1 光盘 光盘技术最终可能使磁盘和磁带存储淘汰。用这种技术，磁存储器所用的读/写头被两束激光代替。一束激光通过在光盘上刻制微小的凹点，对记录表面进行写；而另一束激光用来从光敏感的记录表面读取数据。由于光束容易被偏转到光盘上所需要的位置，所以不需要存取臂。 对用户而言，光盘正成为最有吸引力的选择。它们(光盘)对环境变化不太敏感，并且它们以每兆字节比磁盘低得多的存储器价格提供更多的直接存取存储器。光盘技术仍在出现，并且还需要稳定；然而，目前有三种主要类型的光盘。它们是CD-ROM、WORM盘和磁光盘。 CD-ROM 1980年引入的，非常成功的CD，或紧密盘是设计来提高音乐的录音重放质量的光盘。为了制作一张CD，把音乐的模拟声音转换成等价的数字声音，并且存储在一张4.72英寸的光盘上。在每张光盘上可以用数字格式（用20亿数字位）记录74分钟的音乐。因为它的巨大存储容量，计算机工业的企业家们立刻认